A spectrometer having a function of tandem mass spectrometry has prevailed recently, which analyzes a sample (a parent ion) after the first dissociation and continues mass spectrometry for dissociated ions after the second or more dissociation. An objective for utilizing a tandem mass spectrometer is to improve the accuracy of identifying a sample by analyzing mass spectrometric data obtained by a mass spectrometer. An analysis of multiple-stage dissociation, which analyzes mass spectrometric data of a parent ion (MS data), another mass spectrometric data (MS.sup.2 data) of dissociated ions of the parent ion and the other mass spectrometric data (MS.sup.3) obtained by further dissociating the dissociated ions, can improve the accuracy of estimation for structure of the parent ion.
Methods for providing the estimated structure of a parent ion, which use mass spectrometric data, are categorized as follows:
(1) method for retrieving a database of mass spectrometric data for a parent ion (MS data)
(2) method for retrieving a database of mass spectrometric data for a parent ion and dissociated ions thereof (MS data and MS.sup.2 data)
(3) method for employing measured mass spectrometric data for a parent ion and dissociated ions thereof (MS data and MS.sup.2 data) but not utilizing database search
As an example of the conventional method (2), the Japanese Published Patent Application 8-124519 discloses a method for determining a candidate for parent ion. The method has the steps of picking up candidates for an ion species, which have peaks correlating respectively with those of mass spectrum of the ion species, referring to a database of peaks; picking up candidates for a desorptive base which have desorptive masses correlating with those of the ion species, referring to a database of desorptive bases; and determining a candidate for the parent ion referring to a database which stores regulations applied to construction of the parent ion from dissociated ions and desorptive bases. It is noted that a tandem mass spectrometric data includes up to MS.sup.2 but not MS.sup.3 or more.
Also as an example of the conventional method (3), there is a computer program called “SeqMS” for supporting an analysis for amino acid sequence developed by Osaka University in Japan, which is reported in Lectures on Experiment in Proteome Analysis Method P137 to P139. The computer program is able to support in identifying amino acid sequences for a peptide without database search, which includes about ten amino acid sequences. The method applied to the program, which employs a statistical processing that takes into account a weighted value of dissociation probability empirically obtained from the mass spectrometric data of a peptide ion and its dissociated ions, provides candidates for the amino acid sequence.
If a mass spectrometer is able to perform mass spectrometry MS.sup.N (N equal to or grater than 3), it is impossible to analyze the obtained mass spectrometric data by database search shown in the above-mentioned conventional methods (1) and (2) since the data base does not cover mass spectrometric data for MS.sup.N (N≧3).
It is also difficult for the method (3), which does not use database search, to improve the accuracy of identifying a parent ion. The reason for it is that the empirical weighting of dissociation probability can not be applied to the mass spectrometric data MS.sup.N (N≧3).